Electrode, method for producing said electrode, electricity storage device provided with said electrode, and conductive carbon mixture for electricity storage device electrode

ABSTRACT

Provided is an electrode which gives an electric storage device that has high energy density and good cycle life. This electrode for an electric storage device is characterized by having an active material layer that contains: an electrode active material particle; and a paste-like conductive carbon that is derived from an oxidized carbon obtained by giving an oxidizing treatment to a carbon raw material with an inner vacancy and covers a surface of the electrode active material particle. The paste-like conductive carbon derived from the oxidized carbon is densely filled not only into a gap that is formed between the electrode active material particles adjacent to each other but also into a pore that exists on the surface of the active material particle, so that the electrode density is increased, thereby improving the energy density of the electric storage device. In addition, since the paste-like conductive carbon suppresses dissolution of the active material, the cycle characteristics of the electric storage device are improved.

TECHNICAL FIELD

The present invention relates to an electrode that leads to an electricstorage device with a high energy density and excellent cycle life. Thisinvention also relates to a method for producing this electrode and forproducing an electric storage device provided with this electrode. Thepresent invention further relates to a conductive carbon mixture that isused for producing the electrode of an electric storage device.

THE RELATED ART

An electric storage device such as a secondary battery, an electricdouble layer capacitor, a redox capacitor and a hybrid capacitor is adevice that is under consideration for wider application as a batteryfor an information device including a cellphone and a notebook-sizedpersonal computer, for a motor drive power supply of a low-emissionvehicle such as an electric vehicle and a hybrid vehicle, and for anenergy recovery system, etc. In these devices, improvements in energydensity and cycle life are desired to meet the requirements of higherperformance and downsizing.

In these electric storage devices, an electrode active material thatrealizes its capacity by a faradaic reaction involving the transfer ofan electron with an ion in an electrolyte (including an electrolyticsolution) or by a nonfaradaic reaction not involving the transfer of anelectron is used for energy storage. Further, this active material isgenerally used in the form of a composite material with anelectroconductive agent. As the electroconductive agent, conductivecarbon such as carbon black, natural graphite, artificial graphite, andcarbon nanotube is generally used. This conductive carbon, usedconcurrently with a low conductive active material, serves to addconductivity to the composite material, and furthermore, acts as amatrix to absorb the volume change in accordance with the reaction ofthe active material. Also, it serves to ensure an electron conductingpath when the active material is mechanically damaged.

The composite material of the active material and the conductive carbonis generally manufactured by a method of mixing particles of the activematerial and the conductive carbon. The conductive carbon does not makea significant contribution to the improvement of the energy density ofan electric storage device, so the quantity of the conductive carbon perunit volume needs to be decreased and that of the active material needsto be increased to obtain an electric storage device with a high energydensity. Therefore, consideration is given to a method to decrease thedistance between the particles of the active material to increase thequantity of the active material per unit volume by improving thedispersibility of the conductive carbon or by reducing the structure ofthe conductive carbon.

For example, Patent Document 1 (JP 2004-134304 A) discloses a nonaqueoussecondary battery that is equipped with a positive electrode thatcontains a small-sized carbon material having an average primaryparticle diameter of 10 to 100 nm (in its example, acetylene black) andthat has a degree of blackness of 1.20 or more. A coating material usedto form the positive electrode is obtained either by dispersing amixture of an active material for a positive electrode, theabovementioned carbon material, a binder and a solvent by a high sheardispersing machine such as a high speed rotational homogenizerdispersing machine or a planetary mixer with three or more rotary axes,or by adding a dispersion body, in which a mixture of the abovementionedcarbon material, a binder and a solvent are dispersed by a high sheardispersing machine, into a paste in which a mixture of the activematerial for a positive electrode, a binder and a solvent are dispersed,and further dispersing. By using the device that has a high shearingforce, the carbon material, which is hard to disperse because of itssmall particle size, becomes evenly dispersed.

Also, Patent Document 2 (JP 2009-35598 A) discloses an electroconductiveagent for an electrode for a nonaqueous secondary battery that consistsof acetylene black whose BET-specific surface area is 30 to 90 m²/g,dibutylphthalate (DBP) oil absorption quantity is 50 to 120 mL/100 g,and pH is 9 or more. The electrode for the secondary battery is formedby dispersing a mixture of this acetylene black and an active materialin a fluid containing a binder to prepare slurry, and applying thisslurry on a current collector and drying it. Since the acetylene blackwith the abovementioned characteristics has a smaller structure comparedwith Ketjen Black or other conventional acetylene blacks, the bulkdensity of a mixture of the acetylene black and the active material isimproved and the battery capacity is improved.

Moreover, it is also considered that an even distribution state of anactive material and a conductive carbon is created by covering a part orall of the surface of the particles of the active material so that theconductive property among the active materials is increased anddegradation of the cycle life is prevented. For example, Patent Document3 (JP H11-283623 A) discloses a method in which base particles of alithium composite oxide such as LiCoO₂, which act as an active material,and sub-particles of a carbon material such as acetylene black, whichact as a conductive agent, are blended while compressive and shearingactions are applied so that a part or all of the surface of the baseparticles of the composite oxide is covered with the sub-particles ofthe carbon material. Patent document 3 also discloses that the compositematerial obtained by this method is used for a positive electrode of anonaqueous secondary battery.

PRIOR ARTS DOCUMENTS Patent Documents

Patent Document 1: JP 2004-134304 A

Patent Document 2: JP 2009-35598 A

Patent Document 3: JP H11-283623 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Further improvement of an electric storage device in terms of energydensity is always desired. However, the inventors have examined theprior arts and found that even by the methods disclosed in PatentDocuments 1 and 2, it is difficult to enable conductive carbon toinfiltrate efficiently between particles of an active material, andtherefore, it is difficult to shorten the distance between the activematerial particles and increase the amount of the active material perunit volume. Therefore, the inventors have found that there is alimitation to the improvement of the energy density with a positiveelectrode and/or a negative electrode using the composite material ofparticles of an active material and conductive carbon. Also, there was alimit to the increase in energy density that could be obtained even by amethod of using carbon particles to cover the surface of particles of anactive material as shown in Patent Document 3, and a satisfactory cyclelife could not be obtained because of reasons such as the dissolution ofactive materials into an electrolytic solution of a nonaqueous secondarybattery.

Therefore, the objective of the present invention is to provide anelectrode that leads to an electric storage device with a high energydensity and excellent cycle life.

Means for Solving Problems

After a keen examination, the inventors have found that theabovementioned purpose is achieved by composing an electrode of anelectric storage device by using an electrode material comprising anoxidized carbon, which is obtained by giving a strong oxidizingtreatment to a carbon raw material with an inner vacancy, and particlesof an active material.

Therefore, the present invention, first of all, relates to an electrodefor an electric storage device having an active material layercomprising:

an electrode active material particle; and

a paste-like conductive carbon that is derived from an oxidized carbonobtained by giving an oxidizing treatment to a carbon raw material withan inner vacancy and covers a surface of the electrode active materialparticle.

The inner vacancy includes a pore in porous carbon powder as well as ahollow of Ketjen Black, an internal or interstitial pore of a carbonnanofiber or a carbon nanotube. The word “paste-like” refers to acondition in which the grain boundary of carbon primary particles is notobserved and non-particulate amorphous carbons are connected with eachother in an SEM image at a magnification of 25,000.

The oxidized carbon obtained by giving an oxidizing treatment to acarbon raw material with an inner vacancy is easily attachable to thesurface of the particles of the active material. Moreover, the oxidizedcarbon obtained by giving a strong oxidizing treatment is compressedintegrally and spreads in a paste-like manner when pressure is appliedto it, and is hard to separate. Therefore, when the oxidized carbonobtained by giving a strong oxidizing treatment and the particles of theactive material are blended to obtain an electrode material for anelectrode of an electric storage device, the oxidized carbon is attachedto and covers the surface of the particles of the active material in theprocess of blending and the dispersity of the particles of the activematerial is improved. Also, by the pressure applied to the oxidizedcarbon in the process of blending, at least part of the oxidized carbonspreads in a paste-like manner and the surface of the particles of theactive material becomes partially covered. Moreover, when an activematerial layer is formed with this electrode material on a currentcollector of an electrode and pressure is applied to the active materiallayer, the oxidized carbon, at least part of which is gelatinized,further spreads and has become dense while covering the surface of theparticles of the active material, the particles of the active materialapproach each other, and accordingly the oxidized carbon which isgelatinized is pushed out not only into gaps that are formed between theactive material particles adjacent to each other, but also into poresthat exist on the surface of the particles of the active material(including gaps between primary particles that are found in secondaryparticles) and densely fills the gaps and pores while covering thesurface of the particles of the active material (see FIG. 2). Therefore,the amount of the active material per unit volume in the electrode isincreased and electrode density is increased. Moreover, the paste-likeoxidized carbon that is densely filled has sufficient conductivity toserve as a conductive agent. The electrode of the present invention maycomprise the oxidized carbon which is not gelatinized.

In a preferred embodiment of the electrode of the present invention, thepaste-like conductive carbon that is derived from the oxidized carbonalso exists inside a gap that is formed between the electrode activematerial particles adjacent to each other and/or a pore that exists onthe surface of the electrode active material particle, the gap and thepore having a width of 50 nm or less. Therefore, the coverage of thesurface of the particles of the active material by the paste-likeconductive carbon is improved, the conductive property of the entireactive material layer is improved, and the electrode density is alsoimproved. The term “the width of the gap formed between the electrodeactive material particles adjacent to each other” means the shortestdistance of the distances between the adjacent particles of theelectrode active material and the term “the width of the pore thatexists on the surface of the electrode active material” means theshortest distance of the distances between the points at the oppositeends of the pore.

It has been found that, although the electrode of the present inventionhas the active material layer comprising the densely-filled paste-likeconductive carbon, the impregnation of an electrolytic solution to theelectrode in an electric storage device is not inhibited. It has alsobeen found by measuring the pore distribution of the active materiallayer of the electrode by a mercury intrusion method that the activematerial layer has pores with a diameter of 5 to 40 nm in a preferredembodiment of the electrode of the present invention. These pores areconsidered to be mostly the pores in the paste-like conductive carbonthat is derived from an oxidized carbon and has become dense. Thesepores are so large that the electrolytic solution in the electricstorage device passes through the paste-like conductive carbon andreaches the particles of the active material. Therefore, the paste-likeconductive carbon in the electrode has a sufficient conductive propertyand does not inhibit the impregnation of the electrolytic solution inthe electric storage device. As a result, the energy density of theelectric storage device is improved.

Moreover, it has been found that, probably because the surface of theparticles of the active material is covered by the dense, paste-likeconductive carbon up to the inner side of the pores that exist on thesurface of the particles of the active material in the active materiallayer of the electrode of the present invention, dissolution of theactive material in the electrolytic solution is inhibited and the cyclecharacteristic of the electric storage device is improved, although theimpregnation of the electrolytic solution to the electrode in theelectric storage device is not inhibited.

It is preferable that the oxidized carbon that leads to theabovementioned past-like conductive carbon comprises a hydrophilic partand that the contained amount of the hydrophilic part is 10% by mass ormore of the entire oxidized carbon. The “hydrophilic part” of carbonmeans the following: 0.1 g of carbon is added to 20 ml of an ammoniaaqueous solution with pH 11, ultrasonic irradiation is applied for 1minute, and a fluid obtained is left for 5 hours to precipitate itssolid phase part. The part that does not precipitate and is dispersed inthe ammonia aqueous solution with pH 11 is the “hydrophilic part.” Also,the contained amount of the hydrophilic part in the entire carbon can becalculated by the following method: After the precipitation of the solidphase part, the supernatant fluid is removed, the remaining part isdried, and the weight of the solid object after drying is measured. Theweight calculated by subtracting the weight of the solid object afterdrying from the weight of the initial carbon (0.1 g) is the weight ofthe “hydrophilic part,” which is dispersed in the ammonia aqueoussolution with pH 11. The weight ratio of the weight of the “hydrophilicpart” against the weight of the initial carbon (0.1 g) is the containedamount of the “hydrophilic part” in the carbon.

The ratio of the hydrophilic part in conductive carbon such as carbonblack, natural graphite and carbon nanotube, which is used as aconductive agent in an electrode of a conventional electric storagedevice, is 5% by mass or less of the entire conductive carbon. However,by using a carbon having an inner vacancy as a raw material and givingan oxidizing treatment to this raw material, the surface of itsparticles is oxidized and a hydroxy group, a carboxy group and an etherbond are introduced into the carbon, and a conjugated double bond of thecarbon is oxidized so that a carbon single bond is formed, acarbon-carbon bond is partially severed, and a hydrophilic part isformed on the surface of the particles. Then, as the intensity of theoxidizing treatment is increased, the percentage of the hydrophilic partin the carbon particle is increased and the hydrophilic part accountsfor 10% by mass or more of the entire carbon. Moreover, such an oxidizedcarbon is likely to be compressed integrally and spread in a paste-likemanner when pressure is applied to it, cover most or all of the surfaceof the particles of the active material up to the inside of the poresthat exist on the surface of the particles of the active material, andbecome dense. As a result, an electrode in which 80% or more, preferably90% or more and especially preferably 95% or more of the surface (outersurface) of the particles of the active material in the active materiallayer of the electrode contacts the paste-like conductive carbon, whichis derived from the oxidized carbon and become dense, can be obtained.The coverage rate of the surface of the particles of the active materialby the paste-like conductive carbon is a value calculated by observationof SEM images of the cross-sectional surface of the active materiallayer at a magnification of 25,000.

In the electrode for an electric storage device of the presentinvention, it is preferable that the electrode active material particlesin the active material layer are composed of fine particles with anaverage diameter of 0.01 to 2 μm that are operable as a positiveelectrode active material or a negative electrode active material andgross particles with an average diameter of more than 2 μm and not morethan 25 μm that are operable as an active material of the same electrodeas the fine particles. The gross particles increase the electrodedensity on their own and also have an effect of suitably pressing theoxidized carbon at the time of producing an electrode material andproducing an electrode, swiftly transforming the oxidized carbon topaste and making it dense, and therefore increasing the electrodedensity and improving the energy density of the electric storage device.Also, due to the pressure applied to the active material layer inproducing the electrode, the fine particles press the oxidized carbon,at least part of which is gelatinized, and are pushed out into the gapsthat are formed between the adjacent gross particles together with thepaste-like oxidized carbon, so that the electrode density furtherincreases and the energy density of the electric storage device furtherimproves. The average diameter of the active material particles is the50% radius (median diameter) as in the measurement of particle sizedistribution obtained by using a light scattering particle size meter.

In the electrode for an electric storage device of the presentinvention, it is preferable that a different kind of conductive carbon,especially conductive carbon that has a higher electroconductivity thanthe paste-like conductive carbon that is derived from the oxidizedcarbon, is further comprised in the active material layer. When pressureis applied to the active material layer when the electrode is produced,this carbon is also covered by the paste-like conductive carbon anddensely fills the gaps formed by the adjacent particles of the activematerial together with the paste-like conductive carbon and theconductivity of the active material layer is improved, so that theenergy density of the electric storage device further improves.

In the electrode for an electric storage device of the presentinvention, it is preferable that the mass ratio of the electrode activematerial particle and the conductive carbon in the active material layeris within the range of 95:5 to 99:1. If the different conductive carbonis comprised in addition to the paste-like conductive carbon that isderived from the oxidized carbon, it is preferable that the mass ratioof the total amount of these carbons and the electrode active materialparticle is within the abovementioned range. If the ratio of theconductive carbon is lower than the abovementioned range, the conductiveproperty of the active material layer tends to become insufficient, andthe cycle characteristic tends to decrease because the coverage rate ofthe conductive carbon over the active material particles decreases. Ifthe ratio of the conductive carbon is larger than the abovementionedrange, the electrode density tends to decrease and the energy density ofthe electric storage device tends to decrease.

As mentioned above, the electrode for an electric storage device of thepresent invention can be suitably produced by a method comprising:

a preparation process of blending the electrode active material particleand the oxidized carbon obtained by giving an oxidizing treatment to acarbon raw material with an inner vacancy so that an electrode material,in which at least part of the oxidized carbon is gelatinized andattached to the surface of the electrode active material particle, isprepared; and

a pressure process of forming the active material layer on a currentcollector with the electrode material and applying pressure to theactive material layer.

Therefore, the present invention also relates to the method forproducing the electrode.

If a different conductive carbon, especially a conductive carbon thathas a higher conductivity than the paste-like conductive carbon derivedfrom the oxidized carbon is comprised in the active material layer, itis preferable that the abovementioned preparation process is conductedby a process comprising:

a step of dry blending the oxidized carbon and the different conductivecarbon so that a conductive carbon mixture, in which at least part ofthe oxidized carbon is gelatinized and is attached to the surface of thedifferent conductive carbon, is obtained; and

a step of dry blending or wet blending the conductive carbon mixture andthe electrode active material particle so that the oxidized carbon, atleast part of which is gelatinized, is also attached to the surface ofthe electrode active material particle.

Fine carbon particles are poorly compatible with a binder and a solvent.Therefore, when an electrode material in the form of slurry containing abinder and a solvent is prepared, wet blending by using a high sheardispersing device, or a blending method in which the particles of anelectrode active material and carbon are dry blended and then a binderand a solvent are added and wet blended, are commonly used as mentionedabove in relation to Patent Document 1 and Patent Document 2. However,the abovementioned conductive carbon mixture produces an electrode witha high electrode density and excellent conductive property irrespectiveof the blending method of the conductive carbon mixture and theparticles of the electrode active material. Therefore, the presentinvention also relates to a conductive carbon mixture for producing anelectrode for an electric storage device, in which the conductive carbonmixture comprises: a conductive carbon, at least part of which isgelatinized, which is derived from an oxidized carbon obtained by givingan oxidizing treatment to a carbon raw material with an inner vacancy;and a conductive carbon different from the oxidized carbon, and theconductive carbon, at least part of which is gelatinized, is attached toa surface of the different conductive carbon.

The electrode of the present invention leads to an electric storagedevice with a high energy density and excellent cycle life. Therefore,the present invention also relates to an electrode device with theabovementioned electrode.

Advantageous Effects of the Invention

In the electrode of the present invention that has an active materiallayer comprising an electrode active material particle and a paste-likeconductive carbon that is derived from an oxidized carbon obtained bygiving an oxidizing treatment to a carbon raw material with an innervacancy and covers a surface of the electrode active material particle,the paste-like conductive carbon that is derived from the oxidizedcarbon is densely filled not only into the gaps formed between theparticles of an active material but also into the inside of the poresthat exist on the surface of the particles of the active material, sothat the amount of the active material per unit volume in the electrodeis increased, and the electrode density is increased. Also, thepaste-like conductive carbon that is densely filled has a sufficientconductive property to function as a conductive agent and does notinhibit the impregnation of the electrolytic solution in the electricstorage device. Therefore, the energy density of the electric storagedevice is improved. Also, probably because the surface of the activematerial particles is covered by the oxidized carbon that spreads in apaste-like manner up to the inside of the pores that exist on thesurface of the active material particles in the electrode of the presentinvention, dissolution of the active material in the electrolyticsolution is inhibited and the cycle characteristic of the electricstorage device is improved when the electrode of the present inventionis used in the electric storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph in which the relationship between the containedamount of a hydrophilic part of an oxidized carbon and the electrodedensity is shown for electrodes of working examples and comparativeexamples.

FIG. 2 shows SEM images of the cross-section of an electrode of aworking example: (A) at a magnification of 1,500 and (B) at amagnification of 25,000.

FIG. 3 shows SEM images of the cross-section of an electrode of acomparative example: (A) at a magnification of 1,500 and (B) at amagnification of 25,000.

FIG. 4 shows a graph that shows the result of measuring the distributionof pores in electrodes shown in FIGS. 2 and 3 by the mercury intrusionmethod.

FIG. 5 shows the rate characteristics of a lithium ion secondary batterythat has an electrode of a working example or a comparative example.

FIG. 6 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 5.

FIG. 7 shows the rate characteristics of a lithium ion secondary batterythat has an electrode of a working example or a comparative example.

FIG. 8 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 7.

FIG. 9 shows the rate characteristics of a lithium ion secondary batterythat has an electrode of a working example or a comparative example.

FIG. 10 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 9.

FIG. 11 shows an SEM image at a magnification of 50,000 of a mixturethat is obtained by dry blending of an oxidized carbon, acetylene blackand particles of an active material.

FIG. 12 shows the rate characteristics of a lithium ion secondarybattery that has an electrode of a working example or a comparativeexample.

FIG. 13 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 12.

FIG. 14 shows SEM images at a magnification of 50,000 of a mixture thatis obtained by dry blending of an oxidized carbon or acetylene black andfine particles of an active material.

FIG. 15 shows SEM images at a magnification of 100,000 of a mixture thatis obtained by dry blending of an oxidized carbon or acetylene black andgross particles of an active material.

FIG. 16 shows a SEM image at a magnification of 50,000 of a conductivecarbon mixture.

FIG. 17 shows TEM images of a conductive carbon mixture: (A) at amagnification of 100,000 and (B) at a magnification of 500,000.

FIG. 18 shows a SEM image at a magnification of 25,000 of thecross-section of the electrode of a working example that is manufacturedwith a conductive carbon mixture.

FIG. 19 shows a graph that shows the result of measuring the DC internalresistance of a lithium ion secondary battery in which an electrode of aworking example or a comparative example is used.

DETAILED DESCRIPTION OF THE INVENTION

An electrode for an electric storage device of the present invention hasan active material layer that comprises an electrode active materialparticle and a paste-like conductive carbon that is derived from anoxidized carbon obtained by giving an oxidizing treatment to a carbonraw material with an inner vacancy and covers a surface of the electrodeactive material particle. The oxidized carbon before it is gelatinizedis described first, and then the electrode of the present invention andan electric storage device provided with the electrode is described.

(1) Oxidized Carbon

In the electrode of the present invention, the oxidized carbon thatleads to the paste-like conductive carbon comprised in the activematerial layer is produced by using as a raw material a carbon with aninner vacancy such as porous carbon powder, Ketjen Black, furnace blackwith pores, carbon nanofiber and carbon nanotube. It is preferable touse a carbon with an inner vacancy whose specific surface area asmeasured by the BET method is 300 m²/g or more as a carbon raw materialbecause the carbon is prone to become an oxidized carbon that isgelatinized by oxidizing treatment. Above all, spherical particles suchas Ketjen Black and furnace black with pores are preferable. An oxidizedcarbon that is gelatinized is hard to be obtained even if a solid carbonis used as a raw material and oxidizing treatment is given.

A heretofore-known oxidizing treatment can be used without anyrestriction for oxidizing treatment of the carbon raw material with aninner vacancy. For example, the oxidized carbon can be obtained bytreating the carbon raw material in a solution of acid or hydrogenperoxide. For the acid, nitric acid, a mixture of nitric acid andsulfuric acid, and hypochlorous acid aqueous solution can be used. Also,the oxidized carbon can be obtained by heating the carbon raw materialin an oxygen-containing atmosphere, water vapor or carbon dioxide.Moreover, the oxidized carbon can be obtained by plasma treatment,ultraviolet irradiation, corona discharge treatment, and glow dischargetreatment of the carbon raw material in an oxygen-containing atmosphere.

When the oxidizing treatment is applied to the carbon raw material withan inner vacancy, the surface of its particle is oxidized, and a hydroxygroup, a carboxy group and an ether bond are introduced into the carbon,and a conjugated double bond of the carbon is oxidized so that a carbonsingle bond is formed, a carbon-carbon bond is partially severed, and apart having high hydrophilicity is formed on the surface of theparticle. The oxidized carbon with such hydrophilic part easily attachesto the surface of the particles of an active material and aggregation ofthe particles of an active material is effectively inhibited. Then, asthe intensity of the oxidizing treatment is increased, the ratio of thehydrophilic part in the carbon particle is increased and the oxidizedcarbon that is gelatinized in the process of producing an electrode isobtained. It is preferable that the contained amount of the hydrophilicpart in the oxidized carbon is 10% by mass or more of the entireoxidized carbon. It is especially preferable that the contained amountof the hydrophilic part in the oxidized carbon is 12% by mass or moreand 30% by mass or less of the entire oxidized carbon.

The oxidized carbon having a hydrophilic part that account for 10% bymass or more of the entire oxidized carbon can be suitably obtained bythe manufacturing method comprising:

(a) a process in which acidic treatment is given to a carbon rawmaterial with an inner vacancy;(b) a process in which the product after the acidic treatment and atransition metal compound are mixed;(c) a process in which the mixture obtained is pulverized to produce amechanochemical reaction;(d) a process in which the product after the mechanochemical reaction isheated in a nonoxidizing atmosphere; and(e) a process in which the aforementioned transition metal compoundand/or its reaction product is removed from the product after heating.

In the (a) process, the carbon raw material with an inner vacancy,preferably Ketjen Black, is left immersed in acid. An ultrasonic wavecan be irradiated during this immersion. As acid, an acid usually usedfor an oxidizing treatment of carbon such as nitric acid, a mixture ofnitric acid and sulfuric acid, and an aqueous solution of hypochlorousacid can be used. The immersion time depends on the concentration ofacid or the quantity of the carbon raw material to be treated, and isusually within the range of 5 minutes to 5 hour. The carbon after acidictreatment is sufficiently washed by water and dried, and then mixed withthe transition metal compound in the (b) process.

For the chemical compound of transition metal to be added to the carbonraw material in the (b) process, an inorganic metallic salt oftransition metal such as a halide, nitrate, sulfate and carbonate; anorganic metallic salt of transition metal such as formate, acetate,oxalate, methoxide, ethoxide and isopropoxide; or a mixture thereof canbe used. These chemical compounds can be used alone, or two or morekinds can be used as a mixture. Chemical compounds that containdifferent transition metals can be mixed in a prescribed amount andused. Also, a chemical compound other than the chemical compound oftransition metal, such as an alkali metal compound, can be addedconcurrently unless it has an adverse effect on the reaction. Since theoxidized carbon is mixed with particles of an active material and usedin manufacturing an electrode of an electric storage device, it ispreferable that a chemical compound of an element constituting theactive material is added to the carbon raw material so that adulterationof an element that can serve as impurities against the active materialcan be prevented.

In the (c) process, the mixture obtained in the (b) process ispulverized and a mechanochemical reaction is produced. Examples of apowdering machine for this reaction are a mashing machine, stone mill,ball mill, bead mill, rod mill, roller mill, agitation mill, planetarymill, vibrating mill, hybridizer, mechanochemical composite device andjet mill. Milling time depends on the powdering machine used or thequantity of the carbon to be treated and has no strict restrictions, butis generally within the range of 5 minutes to 3 hours. The (d) processis conducted in a nonoxidizing atmosphere such as a nitrogen atmosphereand an argon atmosphere. The temperature and time of heating is chosenin accordance with the chemical compound of transition metal used. Inthe subsequent (e) process, the oxidized carbon having a hydrophilicpart that account for 10% by mass or more of the entire oxidized carboncan be obtained by removing the chemical compound of transition metaland/or its reaction product from the product that has been heated bymeans of acid dissolution etc., then sufficiently washing and drying.

In the manufacturing method, the chemical compound of transition metalpromotes the oxidation of the carbon raw material by the mechanochemicalreaction in the (c) process, and the oxidation of the carbon rawmaterial rapidly proceeds. By this oxidation, the oxidized carbon thatcomprises a hydrophilic part, which is 10% by mass or more of the entireoxidized carbon, can be obtained.

The oxidized carbon having a hydrophilic part that accounts for 10% bymass or more of the entire oxidized carbon can be obtained by giving astrong oxidizing treatment to a carbon raw material with an innervacancy, and it is also possible to facilitate the oxidation of thecarbon raw material with an inner vacancy by a method other than theabovementioned method of production.

The oxidized carbon obtained is used for an electrode of an electricstorage device such as a secondary battery, an electric double layercapacitor, a redox capacitor and a hybrid capacitor in an embodiment inwhich the oxidized carbon is mixed with an electrode active materialthat realizes its capacity by a faradaic reaction that involves thetransfer of an electron between an ion in an electrolyte of an electricstorage device or a nonfaradaic reaction that does not involve thetransfer of an electron.

(2) Electrode

The electrode for an electric storage device of the present inventioncan be suitably obtained by a method of production that comprises:

(A) a preparation process of blending an electrode active materialparticle and the oxidized carbon so that an electrode material, in whichat least part of the oxidized carbon is gelatinized and attached to thesurface of the electrode active material particle, is prepared; and(B) a pressure process of forming an active material layer on a currentcollector with the electrode material and applying pressure to theactive material layer.

In process (A), aggregation of the active material particles can beinhibited because the oxidized carbon is attached to the surface of theactive material particles and covers the surface. Also, by the pressureapplied to the oxidized carbon in the process of blending, at least partof the oxidized carbon spreads in a paste-like manner and the surface ofthe active material particles is partially covered.

As the electrode active material used in process (A), an active materialthat is used in a conventional electric storage device as an activematerial for a positive electrode or an active material for a negativeelectrode can be used without any specific restrictions. The activematerial can be a single chemical compound or a mixture of two or morekinds of chemical compound.

Examples of a positive electrode active material for a secondary batteryare, among all, LiMO₂ having a laminar rock salt structure, laminarLi₂MnO₃-LiMO₂ solid solution, and spinel LiM₂O₄ (M in the formulasignifies Mn, Fe, Co, Ni or a combination thereof). Specific examples ofthese are LiCoO₂, LiNiO₂, LiNi_(4/5)Co_(1/5)O₂,LiNi_(1/3)CO_(1/3)Mn_(1/3)O₂, LiNi_(1/2)Mn_(1/2)O₂, LiFeO₂, LiMnO₂,Li₂MnO₃—LiCoO₂, Li₂MnO₃—LiNiO₂, Li₂MnO₃—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂,Li₂MnO₃—LiNi_(1/2)Mn_(1/2)O₂,Li₂MnO₃—LiNi_(1/2)Mn_(1/2)O₂—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiMn₂O₄ andLiMn_(3/2)Ni_(1/2)O₄. Other examples include sulfur and a sulfide suchas Li₂S, TiS₂, MoS₂, FeS₂, VS₂ and Cr_(1/2)V_(1/2)S₂, a selenide such asNbSe₃, VSe₂ and NbSe₃, an oxide such as Cr₂O₅, Cr₃O₈, VO₂, V₃O₈, V₂O₅and V₆O₁₃ as well as a complex oxide such asLiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiVOPO₄, LiV₃O₅, LiV₃O₈, MoV₂O₈,Li₂FeSiO₄, Li₂MnSiO₄, LiFePO₄, LiFe_(1/2)Mn_(1/2)PO₄, LiMnPO₄ andLi₃V₂(PO₄)₃.

Examples of a negative electrode active material for a secondary batteryare an oxide such as Fe₂O₃, MnO, MnO₂, Mn₂O₃, Mn₃O₄, CoO, Co₃O₄, NiO,Ni₂O₃, TiO, TiO₂, SnO, SnO₂, SiO₂, RuO₂, WO, WO₂ and ZnO, metal such asSn, Si, Al and Zn, a complex oxide such as LiVO₂, Li₃VO₄ and Li₄Ti₅O₁₂,and a nitride such as Li_(z) Co_(0.4)N, Ge₃N₄, Zn₃N₂ and Cu₃N.

As an active material in a polarizable electrode of an electric doublelayer capacitor, a carbon material with a large specific surface areasuch as activated carbon, carbon nanofiber, carbon nanotube, phenolresin carbide, polyvinylidene chloride carbide and microcrystal carbonis exemplified. In a hybrid capacitor, a positive electrode activematerial exemplified for a secondary battery can be used as a positiveelectrode. In this case, a negative electrode is composed of apolarizable electrode using activated carbon etc. Also, a negativeelectrode active material exemplified for a secondary battery can beused as a negative electrode. In this case, a positive electrode iscomposed of a polarizable electrode using activated carbon etc. As apositive electrode active material of a redox capacitor, a metal oxidesuch as RuO₂, MnO₂ and NiO is exemplified, and a negative electrode iscomposed of an active material such as RuO₂ and a polarizable materialsuch as activated carbon.

There is no restriction as to the shape and particle diameter of theelectrode active material particles that are mixed with the oxidizedcarbon in process (A), but it is preferable that the average particlediameter of the particles of the active material is more than 2 μm andnot more than 25 μm. The particles of the active material that have sucha relatively large diameter improve electrode density on their own, andin the process of blending with the oxidized carbon, gelatinization ofthe oxidized carbon is promoted by the compressing strength of theparticles. Also, in process (B) shown below, during the step of applyingpressure to the active material layer on the current collector, theparticles of the active material that have such a relatively largeparticle diameter further press the oxidized carbon, at least a part ofwhich is gelatinized, and make the oxidized carbon further spread in apaste-like manner, and make the carbon denser. As a result, theelectrode density further increases and the energy density of anelectric storage device further improves.

Also, it is preferable that the active material particles are composedof fine particles with an average diameter of 0.01 to 2 μm and grossparticles with an average diameter of more than 2 μm and not more than25 μm that are operable as an active material of the same electrode asthe fine particles. It is said that particles with a small particlediameter easily aggregate, but the oxidized carbon is attached to andcovers the surface not only of the gross particles but also of the fineparticles, so that aggregation of the particles of the active materialcan be inhibited, and the blending state of the particles of the activematerial and the oxidized carbon can be uniformalized. Also, asmentioned above, the gross particles promote gelatinization anddensification of the oxidized carbon, increase the electrode density,and improve the energy density of the electric storage device. Further,in process (B), during the step of applying pressure to the activematerial layer on the current collector, the fine particles press theoxidized carbon, at least part of which is gelatinized, and are pushedout into and fill the gaps that are formed between the adjacent grossparticles together with the paste-like oxidized carbon, so that theelectrode density further increases and the energy density of theelectric storage device further improves. It is preferable to selectgross particles and fine particles within the range of 80:20 to 95:5 bymass, and it is more preferable to select them within the range of 90:10to 95:5 by mass.

The mass ratio of the electrode active material particles and theoxidized carbon that are used in process (A) is preferably within therange of 90:10 to 99.5:0.5 and more preferably within the range of 95:5to 99:1 in order to obtain an electric storage device with a high energydensity. If the ratio of the oxidized carbon is lower than theabovementioned range, the conductive property of the active materiallayer tends to become insufficient, and the cycle characteristic tendsto decrease because the coverage rate of the oxidized carbon over theactive material particles decreases. Also, if the ratio of the oxidizedcarbon is larger than the abovementioned range, the electrode densitytends to decrease and the energy density of the electric storage devicetends to decrease.

In process (A), in addition to the electrode active material particlesand the oxidized carbon, a conductive carbon different from the oxidizedcarbon, a binder and a solvent for blending can be used as needed, inorder to produce an electrode material, in which at least part of theoxidized carbon is gelatinized and attached to the surface of theelectrode active material particles. By using a solvent, an electrodematerial in the form of slurry can be obtained.

For the different conductive carbon, carbon black such as Ketjen Black,furnace black, acetylene black and channel black, fullerene, carbonnanotube, carbon nanofiber, graphene, amorphous carbon, carbon fiber,natural graphite, artificial graphite, graphitized Ketjen Black,mesoporous carbon, and vapor grown carbon fiber etc., which are used inan electrode in a conventional electric storage device, can be used. Theusage of a conductive carbon that has a higher conductive property thanthe paste-like conductive carbon derived from the oxidized carbon, andespecially the usage of acetylene black, is preferable. Aggregation ofthe different conductive carbon can be inhibited because the oxidizedcarbon is attached to and covers not only the surface of the particlesof the active material, but also the surface of the different conductivecarbon. Moreover, in process (B) shown below, the energy density of theelectric storage device further improves because the differentconductive carbon are pushed out into and fill the gaps that are formedbetween adjacent particles together with the paste-like oxidized carbonduring the step of applying pressure to the active material layer on thecurrent collector and the conductive property of the whole electrode isimproved. The ratio of the oxidized carbon and the different conductivecarbon is preferably within the range of 3:1 to 1:3 by mass and morepreferably within the range of 2.5:1.5 to 1.5:2.5 by mass. Moreover, incases where the different conductive carbon is used, the ratio of theelectrode active material panicles and the total amount of the differentconductive carbon and the oxidized carbon is preferably within the rangeof 10:90 to 0.5:99.5 by mass and more preferably within the range of5:95 to 1:99 by mass.

As the binder, a heretofore known binder such aspolytetrafluoroethylene, polyvinylidene fluoride,tetrafluoroethylene-hexafluoropropylene copolymer, polyvinyl fluorideand carboxymethylcellulose can be used. It is preferable that the amountof the binder used is 1 to 30% by mass of the total amount of theelectrode material. If the amount of the binder used is 1% by mass orless, the strength of the active layer is not sufficient, and if theamount of the binder used is 30% by mass or more, drawbacks such as adecrease in the discharge capacity of an electrode or excessive internalresistance arise. As the solvent for blending, a solvent such asN-methyl pyrrolidone that does not adversely affect the other componentelements in the electrode material can be used without any restriction.There is no restriction as to the amount of the solvent provided eachcomponent element within the mixture is evenly blended. The binder canbe used under the condition of being dissolved in the solvent.

In process (A), there is no restriction as to the method and order ofblending the electrode active material particles and the oxidizedcarbon, and if needed, the conductive carbon different from the oxidizedcarbon, the binder and the solvent for blending.

However, if the conductive carbon different from the oxidized carbon isnot used, it is preferable to dry blend the particles of the electrodeactive material and the oxidized carbon in process (A). By sufficientlykneading the obtained product with a binder as necessary and a solvent,an electrode material in the form of slurry can be obtained. For dryblending, a mashing machine, stone mill, ball mill, bead mill, rod mill,roller mill, agitation mill, planetary mill, vibration mill, hybridizer,mechanochemical composite device and jet mill can be used. Especially,it is preferable to give a mechanochemical treatment to the activematerial particles and the oxidized carbon because the coatability andthe evenness of the coverage of the active material particles by theoxidized carbon are improved. The time for dry blending can varyaccording to the total amount of the active material particles and theoxidized carbon to be blended and the blending device used, butgenerally it is within the range of 1 to 30 minutes. Also, there is nospecial restriction as to the method of kneading with the binder and thesolvent, which can be done manually with a mortar or which can be donewith a heretofore known kneading device such as a stirring machine or ahomogenizer. If each component element in the electrode material isevenly blended, the mixing time can be short.

If the active material particles are composed of fine particles andgross particles, and the different conductive carbon is not used, all ofthe fine particles, the gross particles and the oxidized carbon can beintroduced into a blending device and dry blended in process (A). Bysufficiently kneading the product obtained by dry blending together witha binder as needed and a solvent, an electrode material in the form ofslurry can be obtained. However, it is preferable to conduct the dryblending in the following two steps:

(A1) obtaining a preliminary mixture by dry blending the oxidized carbonand the fine particles, and(A2) dry blending the preliminary mixture obtained and the grossparticles.

Conducting the dry blending by step (A1) and step (A2) is preferablebecause an electrode material, in which the gross particles and fineparticles that are covered by the oxidized carbon are evenly blended ina highly dispersed manner, can be obtained. Moreover, because theproduct obtained in step (A2) is compatible with a binder and a solvent,an electrode material in the form of slurry, in which each componentelement is evenly blended, can be easily obtained. The ratio of the fineparticles and the oxidized carbon in the step of obtaining thepreliminary mixture is preferably within the range of 70:30 to 90:10 bymass and more preferably within the range of 75:25 to 85:15.

If the conductive carbon different from the oxidized carbon is used, allof the active material particles, the oxidized carbon and the differentconductive carbon can be introduced into a blending device and dryblended in process (A). By sufficiently kneading the product obtained bydry blending with a binder as needed and a solvent, an electrodematerial in the form of slurry can be obtained. However, it ispreferable to conduct the dry blending in the following two steps:

(AA1) dry blending the oxidized carbon and the different conductivecarbon, and(AA2) dry blending the mixture obtained in step (AA1) and the activematerial particles.

The mixture obtained in step (AA1) is a “conductive carbon mixture.” Inthis step, the oxidized carbon is attached to the surface of thedifferent conductive carbon, the gelatinization of the oxidized carbonpartially proceeds, and the conductive carbon mixture, in which theoxidized carbon, at least part of which is gelatinized, is attached tothe surface of the different conductive carbon, is obtained. Then, instep (AA2), the oxidized carbon, at least part of which is gelatinized,is also attached to the surface of the electrode active materialparticles, and an electrode, in which the electrode active materialparticles and the different conductive carbon which are covered with theoxidized carbon are evenly blended in a highly dispersed manner, isobtained. Also, because this conductive carbon mixture is compatiblewith a binder and a solvent, one of the following steps can be conductedinstead of step (AA2) and the subsequent kneading with a binder and asolvent in order to obtain an electrode material in the form of slurry:

(aa1) wet blending the conductive carbon mixture, the active materialparticles, the binder and the solvent, or(aa2) wet blending the conductive carbon mixture, the binder and thesolvent, further adding the active material particles and wet blending,or(aa3) wet blending the conductive carbon mixture, the active materialparticles and the solvent, further adding the binder and wet blending.

Fine carbon particles are said to be poorly compatible with a binder anda solvent, but by using the conductive carbon mixture, an electrodematerial in which each component is evenly blended can be easilyobtained regardless of the blending method of this mixture and theelectrode active material particles. Also, various production lines canbe established if the conductive carbon mixture is prepared previouslybecause the subsequent blending of the active material particles and thebinder can be conducted by both wet blending and dry blending.

Also, if the active material particles are composed of fine particlesand gross particles, and the different conductive carbon is used, all ofthe fine particles, the gross particles, the oxidized carbon and thedifferent conductive carbon can be introduced into a blending device anddry blended in process (A). An electrode material in the form of slurrycan be obtained by sufficiently kneading the product obtained by dryblending with a binder as needed and a solvent. However, it ispreferable to conduct the dry blending in the following steps:

(AB1) dry blending the oxidized carbon and the different conductivecarbon,(AB2) dry blending the mixture obtained in step (AB1) and the fineparticles, and(AB3) dry blending the mixture obtained in step (AB2) and the grossparticles.It is also preferable to conduct the dry blending in the followingsteps:(AC1) dry blending the oxidized carbon and the fine particles,(AC2) dry blending the mixture obtained in step (AC1) and the differentconductive carbon, and(AC3) dry blending the mixture obtained in step (AC2) and the grossparticles.

In these methods, at least in one of steps (AB1), (AB2), (AC1) and(AC2), the oxidized carbon is attached to the surface of the fineparticles or the different conductive carbon, and the gelatinization ofthe oxidized carbon partially proceeds, and therefore a mixture in whichthe oxidized carbon, the gross particles, the fine particles and thedifferent carbon are evenly blended in a highly dispersed manner can beobtained. Also, because the conductive carbon mixture obtained in step(AB1) is compatible with a binder and a solvent as mentioned above, oneof the following steps can be conducted in order to obtain an electrodematerial in the form of slurry instead of step (AB2), step (AB3) and thesubsequent kneading with a binder and a solvent:

(ab1) wet blending the conductive carbon mixture, the fine particles,the gross particles, the binder and the solvent, or(ab2) wet blending the conductive carbon mixture, the binder and thesolvent, further adding the fine particles and the gross particles andwet blending, or(ab3) wet blending the conductive carbon mixture, the fine particles,the gross particles and the solvent, further adding the binder and wetblending.

In cases where process (A) is conducted by using these methods, theamount of the different conductive carbon used is chosen so that theratio of the fine particles and the total amount of the oxidized carbonand the different conductive carbon is within the range of 70:30 to90:10 by mass and preferably within the range of 75:25 to 85:15 by mass.

In process (B), an active material layer is formed by applying theelectrode material obtained in process (A) onto a current collector toconstitute a positive electrode or negative electrode of an electricstorage device, this active material layer is dried as necessary,pressure is applied to the active material layer by rolling treatment,and an electrode is obtained. The rolling treatment can be given afterthe electrode material obtained in process (A) has been shaped into apredefined form and press-fitted onto the current collector.

In process (B), as pressure is applied to the active material layer, theoxidized carbon at least part of which is gelatinized further spreadsand has become dense while covering the surface of the active materialparticles, the active material particles approach each other, andaccordingly the paste-like oxidized carbon is pushed out not only intothe gaps formed between the adjacent particles of the active material,but also into the pores that exist on the surface of the particles ofthe active material, and densely fills the gaps and pores while coveringthe surface of the particles of the active material. Therefore, theamount of the active material per unit volume in the electrode isincreased and the electrode density is increased. Moreover, thepaste-like oxidized carbon that is densely filled has a sufficientconductive property to serve as a conductive agent.

As the current collector for an electrode of an electric storage device,an electroconductive material such as platinum, gold, nickel, aluminum,titanium, steel and carbon can be used. For the form of the currentcollector, any form such as a film, foil, plate, net, expanded metal, orcylinder can be adopted. To dry the active material layer, the solventcan be removed, if needed, by heating under reduced pressure. Thepressure applied to the active material layer by rolling treatment isgenerally within the range of 50,000 to 1,000,000 N/cm² and preferablywithin the range of 100,000 to 500,000 N/cm². Also, there is no specialrestriction as to the temperature of the rolling treatment, and therolling treatment can be given at a normal temperature or under aheating condition.

In a preferred embodiment of the electrode of the present invention, thepaste-like conductive carbon in the active material layer also existsinside the gaps formed between the adjacent particles of the electrodeactive material and/or the pores that exist on the surface of theparticles of the electrode active material. Therefore, the coverage ofthe surface of the particles of the active material by the paste-likeconductive carbon is increased, the conductive property of the entireactive material layer is improved and the electrode density is alsoimproved. Conductive carbon such as carbon black, natural graphite andcarbon nanotube, which are used as a conductive agent in an electrode ofa conventional electric storage device, can hardly intrude into gaps orpores of such narrow width.

It has been found that, although the electrode of the present inventionhas the active material layer comprising the paste-like conductivecarbon that is densely filled, impregnation of an electrolytic solutionin an electric storage device into the electrode is not inhibited. In apreferred embodiment of the electrode of the present invention, themeasurement of pore distribution in the active material layer of theelectrode by the mercury intrusion method reveals that the activematerial layer has pores with a diameter of 5 to 40 nm. These finepores, which are considered to be mainly pores in the paste-likeconductive carbon that is derived from the oxidized carbon and that hasbecome dense, are large enough to allow an electrolytic solution in anelectric storage device to pass through the paste-like conductive carbonto the particles of the active material. Therefore, the paste-likeconductive carbon in the electrode has a sufficient conductive propertyand does not inhibit the impregnation of the electrolytic solution inthe electric storage device. As a result, the energy density of theelectric storage device is improved.

Moreover, it has been found that, probably because the surface of theparticles of the active material in the active material layer of theelectrode of the present invention is covered by the dense oxidizedcarbon that spreads in a paste-like manner up to the inside of the poresthat exist on the surface of the particles of the active material,dissolution of the active material in the electrolytic solution isinhibited and the cycle characteristic of the electric storage device isimproved although the impregnation of the electrolytic solution in theelectric storage device into the electrode is not inhibited. In apreferred embodiment of the electrode of the present invention, theamount of dissolution of the active material is decreased by as much as40% or more compared with when an electrode is composed of aconventional conductive agent such as acetylene black and the particlesof the active material. Moreover, because the dissolution of the activematerial is significantly inhibited, the cycle characteristic of theelectric storage device is significantly improved.

Especially, the oxidized carbon with a hydrophilic part that account for10% by mass or more of the whole oxidized carbon is likely to beintegrally compressed and spread in a paste-like manner when pressure isapplied, cover most or all of the surface of the particles of the activematerial up to the inside of the pores that exist on the surface of theparticles of the active material, and become dense. As a result, anelectrode, in which 80% or more, preferably 90% or more and especiallypreferably 95% or more of the surface of the particles of the activematerial in the active material layer of the electrode contacts thepaste-like conductive carbon that is derived from the oxidized carbonand has become dense, is obtained.

(3) Electric Storage Device

The electrode of the present invention is used for an electrode of anelectric storage device such as a secondary battery, an electric doublelayer capacitor, a redox capacitor and a hybrid capacitor. The electricstorage device comprises a pair of electrodes (positive electrode andnegative electrode) and an electrolyte that is placed therebetween asessential components. At least either of the positive electrode or thenegative electrode is produced by the method of producing in the presentinvention.

The electrolyte that is placed between a positive electrode and anegative electrode in an electric storage device can be an electrolyticsolution that is held by a separator, a solid electrolyte, or a gelelectrolyte, that is, an electrolyte that is used in a conventionalelectric storage device can be used without any restrictions.Representative electrolytes are as follows. For a lithium ion secondarybattery, an electrolytic solution in which a lithium salt such as LiPF₆,LiBF₄, LiCF₃SO₃ and LiN(CF₃SO₂)₂ is dissolved in a solvent such asethylene carbonate, propylene carbonate, butylene carbonate anddimethylcarbonate can be used and held by a separator such as polyolefinfiber nonwoven fabric and glass fiber nonwoven fabric. Further, aninorganic solid electrolyte such as Li₅La₃Nb₂O₁₂,Li_(1.5)Al_(0.5)Ti_(1.5)(PO₄)₃, Li₇La₃Zr₂O₁₂ and Li₇P₃S₁₁, an organicsolid electrolyte that is composed of a complex of a lithium salt and amacromolecule compound such as polyethylene oxide, polymethacrylate andpolyacrylate, and a gel electrolyte in which an electrolytic solution isabsorbed into polyvinylidene fluoride and polyacrylonitrile etc. arealso used. For an electric double layer capacitor and a redox capacitor,an electrolytic solution in which a quaternary ammonium salt such as(C₂H₅)₄NBF₄ is dissolved in a solvent such as acrylonitrile andpropylene carbonate is used. For a hybrid capacitor, an electrolyticsolution in which a lithium salt is dissolved in propylene carbonateetc. or an electrolytic solution in which a quaternary ammonium salt isdissolved into propylene carbonate etc. is used.

However, if a solid electrolyte or a gel-like electrolyte is used as theelectrolyte between a positive electrode and a negative electrode, anelectrode material is prepared by adding a solid electrolyte to each ofthe abovementioned component elements in the abovementioned process (A)for the purpose of securing an ion conduction path in the activematerial layer.

EXAMPLES

The present invention is explained in the following examples, though thepresent invention is not limited to the following examples.

(1) Production of Oxidized Carbon and Electrode

Example 1

Ketjen Black (trade name: EC300J, manufacturer: Ketjen BlackInternational Co., the BET specific surface area: 800 m²/g) weighing 10g was added to 300 mL of 60% nitric acid and then the fluid obtained wasirradiated by an ultrasonic wave for 10 minutes, and then the fluid wasfiltered and the Ketjen Black was retrieved. The retrieved Ketjen Blackwas washed with water three times and then dried, so that acid-treatedKetjen Black was obtained. Then, 0.5 g of the acid-treated Ketjen Blackobtained was mixed with 1.98 g Fe(CH₃COO)₂, 0.77 g Li(CH₃COO), 1.10 gC₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31 g H₃PO₄, and 120 mL distilled water,and the mixed fluid obtained was agitated by a stirrer for 1 hour, andthen the mixed fluid was evaporated, dried and solidified at 100° C. inair and a mixture was collected. Then, the mixture obtained wasintroduced into a vibratory ball mill device and pulverization wasconducted at 20 hz for 10 minutes. The powder obtained by pulverizationwas heated at 700° C. for 3 minutes in nitrogen, and a complex in whichLiFePO₄ was supported by Ketjen Black was obtained.

1 g of the complex obtained was added to 100 mL of 30% hydrochloric acidaqueous solution, then the LiFePO₄ in the complex was dissolved byirradiating the fluid obtained with an ultrasonic wave for 15 minutes,and the remaining solid matter was filtered, washed with water anddried. A part of the solid matter after drying was heated to 900° C. inair and its weight loss was measured by TG analysis. Until it wasconfirmed that the weight loss was 100%, that is, no LiFePO₄ remained,the abovementioned process of dissolving LiFePO₄ in the hydrochloricacid aqueous solution, filtering, washing with water and drying wasrepeated, so that an oxidized carbon that did not contain any LiFePO₄was obtained.

Then, 0.1 g of the oxidized carbon obtained was added to 20 ml ofammonia solution with pH 11, and ultrasonic irradiation was conductedfor 1 minute. The fluid obtained was left for 5 hours and a solid phasearea was precipitated. After the precipitation of the solid phase area,the supernatant fluid was removed, the remaining part was dried, and theweight of the solid object after drying was measured. By subtracting theweight of the solid object after drying from the weight of the initialoxidized carbon (0.1 g) and calculating the weight ratio of thesubtracted result against the initial weight of the oxidized carbon (0.1g), the contained amount of the “hydrophilic part” in the oxidizedcarbon was evaluated.

Fe(CH₃COO)₂, Li(CH₃COO), C₆H₈O₇.H₂O, CH₃COOH and H₃PO₄ were introducedinto distilled water, and the compound fluid obtained was agitated by astirrer for 1 hour, and then the compound fluid was evaporated, driedand solidified at 100° C. in air and then heated at 700° C. for 3minutes in nitrogen, and LiFePO₄ fine particles with an initial particlediameter of 100 nm (the average particle diameter: 100 nm) wereobtained. Then, commercially available LiFePO₄ gross particles (initialparticle diameter: 0.5 to 1 μm, secondary particle diameter: 2 to 3 μm,the average particle diameter: 2.5 μm), the fine particles obtained andthe oxidized carbon were mixed at the ratio of 90:9:1, and an electrodematerial was obtained. Then, 5% by mass of the total mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added to the electrode material and kneaded sufficiently so that aslurry was formed, and this slurry was coated on an aluminum foil, driedand then given a rolling treatment, and an electrode with an activematerial layer was obtained. The electrode density of the electrode wascalculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the electrode.

Example 2

The procedure of Example 1 was repeated except that the process in which0.5 g the acid-treated Ketjen Black, 1.98 g Fe(CH₃COO)₂, 0.77 gLi(CH₃COO), 1.10 g C₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31 g H₃PO₄ and 120 mLdistilled water were mixed was changed into a process in which 1.8 g theacid-treated Ketjen Black, 0.5 g Fe(CH₃COO)₂, 0.19 g Li(CH₃COO), 0.28 gC₆H₈O₇.H₂O, 0.33 g CH₃COOH, 0.33 g H₃PO₄ and 250 mL distilled water weremixed.

Example 3

10 g of Ketjen Black used in Example 1 was added to 300 mL of 40% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 10 minutes, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that acid-treated Ketjen Black was obtained. Then,the procedure of Example 2 was repeated except that this acid-treatedKetjen Black 1.8 g was used instead of the acid-treated Ketjen Black 1.8g used in Example 2.

Comparative Example 1

10 g of Ketjen Black used in Example 1 was added to 300 mL of 60% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 1 hour, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that acid-treated Ketjen Black was obtained. Thisacid-treated Ketjen Black was heated at 700° C. for 3 minutes innitrogen. For the oxidized carbon obtained, the contained amount of thehydrophilic part was measured by using the same procedure as theprocedure in Example 1. Also, using the oxidized carbon obtained, anelectrode containing LiFePO₄ was produced by using the same procedure asthe procedure in Example 1 and its electrode density was calculated.

Comparative Example 2

10 g of Ketjen Black used in Example 1 was added to 300 mL of 30% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 10 minutes, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that acid-treated Ketjen Black was obtained. Then,without pulverization by vibratory ball mill, it was heated at 700° C.for 3 minutes in nitrogen. For the oxidized carbon obtained, thecontained amount of the hydrophilic part was measured by using the sameprocedure as the procedure in Example 1. Also, using the oxidized carbonobtained, an electrode containing LiFePO₄ was produced by using the sameprocedure as the procedure in Example 1 and its electrode density wascalculated.

Comparative Example 3

To confirm the contribution of the hydrophilic part to electrodedensity, 40 mg of the oxidized carbon of Example 1 was added to 40 mL ofpure water, ultrasonic irradiation was applied for 30 minutes todisperse the carbon in the pure water, then the dispersion was left for30 minutes, after which the supernatant fluid was removed, then theremaining part was dried, and a solid object was obtained. For thissolid object, the contained amount of the hydrophilic part was measuredby using the same procedure as the procedure in Example 1. Also, usingthe solid object obtained, an electrode containing LiFePO₄ was producedby using the same procedure as the procedure in Example 1 and itselectrode density was calculated.

Comparative Example 4

For the Ketjen Black raw material used in Example 1, the containedamount of the hydrophilic part was measured by using the same procedureas the procedure in Example 1. Also, using the Ketjen Black rawmaterial, an electrode containing LiFePO₄ was produced by using the sameprocedure as the procedure in Example 1 and its electrode density wascalculated.

FIG. 1 is a graph that shows the relationship between the containedamounts of the hydrophilic part in the carbons of Examples 1 to 3 andComparative Examples 1 to 4 and the electrode densities of theelectrodes of Examples 1 to 3 and Comparative Examples 1 to 4. As isevident from FIG. 1, if the contained amount of the hydrophilic partexceeds 8% by mass of the entire oxidized carbon, the electrode densitybegins to increase, and if it exceeds 9% by mass of the entire oxidizedcarbon, the electrode density begins to increase sharply, and if thecontained amount of the hydrophilic part exceeds 10% by mass of theentire oxidized carbon, the high electrode density of 2.6 g/cc or morecan be obtained. Also, as is evident from the comparison of the resultfor Example 1 and the result for Comparative Example 3, the hydrophilicpart of the oxidized carbon largely contributes to the improvement ofelectrode density. Also, based on the observation of SEM images it wasconfirmed that the gelatinization of the oxidized carbon rapidlyprogresses as the contained amount of the hydrophilic part of theoxidized carbon is increased and the electrode density begins toincrease rapidly.

(3) Evaluation as a Lithium Ion Secondary Battery

(i) Active material: LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂

Example 4

Li₂CO₃, Ni(CH₃COO)₂, Mn(CH₃COO)₂ and Co(CH₃COO)₂ were introduced intodistilled water, the compound fluid obtained was agitated by a stirrerfor 1 hour, and then the compound fluid was evaporated, dried andsolidified at 100° C. in air, mixed by using a ball mill, and thenheated at 800° C. in air for 10 minutes, andLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ fine particles with an average diameter of0.5 μm were obtained. These fine particles and the oxidized carbon inExample 1 were mixed at the ratio by mass of 90:10 and a preliminarycompound was obtained. Then, 86% by mass of commercially availableLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ gross particles (average diameter: 5 μm),9% by mass of the abovementioned preliminary compound and 2% by mass ofacetylene black (primary particle diameter: 40 nm) were mixed, and then3% by mass of polyvinylidene fluoride and an adequate quantity ofN-methyl pyrrolidone were added and kneaded sufficiently so that aslurry was formed, and this slurry was coated on an aluminum foil, driedand then given a rolling treatment, and a positive electrode with anactive material layer for a lithium ion secondary battery was obtained.The electrode density of the positive electrode was calculated from themeasured values of the volume and weight of the active material layer onthe aluminum foil in the positive electrode. The value of the electrodedensity was 4.00 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Example 5

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂particles (average diameter: 5 μm), 2% by mass of the oxidized carbon ofExample 1 and 2% by mass of acetylene black (primary particle diameter:40 nm) were mixed, and then 2% by mass of polyvinylidene fluoride and anadequate quantity of N-methyl pyrrolidone were added and kneadedsufficiently so that a slurry was formed, and this slurry was coated onan aluminum foil, dried and then given a rolling treatment, and apositive electrode with an active material layer for a lithium ionsecondary battery was obtained. The electrode density of the positiveelectrode was calculated from the measured values of the volume andweight of the active material layer on the aluminum foil in the positiveelectrode. The value of the electrode density was 3.81 g/cc. Further,using the positive electrode obtained, a lithium ion secondary batterywas manufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

Comparative Example 5

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂particles (average diameter: 5 μm) and 4% by mass of acetylene black(primary particle diameter: 40 nm) were mixed, and then 2% by mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added and kneaded sufficiently so that a slurry was formed, andthis slurry was coated on an aluminum foil, dried and then given arolling treatment, and a positive electrode with an active materiallayer for a lithium ion secondary battery was obtained. The electrodedensity of the positive electrode was calculated from the measuredvalues of the volume and weight of the active material layer on thealuminum foil in the positive electrode. The value of the electrodedensity was 3.40 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

FIG. 2 shows SEM images of the cross-section of the positive electrodeof the lithium ion secondary battery of Example 5 and FIG. 3 shows SEMimages of the cross-section of the positive electrode of the lithium ionsecondary battery of Comparative Example 5. In both figures, (A) is animage at a magnification of 1,500 and (B) is an image at a magnificationof 25,000. In FIG. 2(A) and FIG. 3(A), the thickness of the activematerial layer is shown as t. It can be seen that the active materiallayer in the lithium ion secondary battery of Example 5 is thinner thanthe active material layer in the lithium ion secondary battery ofComparative Example 5, even though the contained amount of the particlesof the active material and the contained amount of carbon in the activematerial layer are the same. Also, from the comparison of FIG. 2(A) andFIG. 3(A), it was found that, in the active material layer of thelithium ion secondary battery of Example 5, the particles of the activematerial approach each other and the ratio of the area occupied bycarbon to the area of the entire active material layer in the image issmall. Further, the forms of carbon in FIG. 2(B) and FIG. 3(B) areremarkably different. In the active material layer of the lithium ionsecondary battery of Comparative Example 5 (FIG. 3(B)), the grainboundaries of carbon (acetylene black) primary particles are clear andthere are large gaps adjacent to the surface boundary between theparticles of the active material and the carbon particles, especiallyadjacent to the pores formed on the surface of the particles of theactive material, in addition to the gaps between the carbon particles,whereas in the active material layer of the lithium ion secondarybattery of Example 5 (FIG. 2(B)), the grain boundaries of carbon primaryparticles are not discernible, the carbon is paste-like and thispaste-like carbon intrudes into the deep parts of the pores of theparticles of the active material that have a width of 50 nm or less (ie.gaps between the primary particles), and gaps are virtually absent.Moreover, it is shown that 90% or more of the surface of the particlesof the active material contact the paste-like carbon. It is assured thatthe difference between the electrode densities in the positiveelectrodes of the lithium ion secondary batteries in Example 5 andComparative Example 5 is derived from the above-described difference inthe forms of carbon.

As mentioned above, because the active material layer in Example 5 isthinner than the active material layer in Comparative Example 5, it isassured that the material filling rate in the former is large, and thematerial filling rate was confirmed in the following formulae. Thetheoretical electrode density refers to the electrode density when gapsin the active material layer are assumed to be 0%.

Material filling rate (%)=electrode density×100/theoretical electrodedensity  (I)

Theoretical electrode density (g/cc)=100/{a/X+b/Y+(100−a−b)/Z}  (II)

where:

a: % by mass of the active material against the entire active materiallayer

b: % by mass of carbon against the entire active material layer

100−a−b: % by mass of polyvinylidene fluoride against the entire activematerial layer

X: true density of the active material

Y: true density of carbon black

Z: true density of polyvinylidene fluoride

As a result, the material filling rate of the active material layer inExample 5 was 86.8% and the material filling rate of the active materiallayer in Comparative Example 5 was 79.1%; in the electrode containingthe paste-like conductive carbon derived from the oxidized carbon, animprovement in the filling rate of as much as 7.7% was observed.

FIG. 4 shows the result of the measurement of the pore distribution inthe active material layer of Example 5 and the active material layer ofComparative Example 5 by the mercury intrusion method. The result showsthat, in the active material layer in Comparative Example 5, pores witha diameter of 20 nm or less are virtually absent, and most of the poresshow peaks at the diameter of approximately 30 nm, the diameter ofapproximately 40 nm, and the diameter of approximately 150 nm.Presumably, the pores that show a peak at the diameter of approximately150 nm are pores that are mainly attributable to the particles of anactive material and the pores that show peaks at the diameter ofapproximately 30 nm and the diameter of approximately 40 nm are thepores that are mainly found between particles of acetylene black. On theother hand, it is assured that, in the active material layer in Example5, the number of pores with a diameter of approximately 100 nm or moreamong the pores in the active material layer in Comparative Example 5 isdecreased, and instead the number of pores with a diameter within therange of 5 to 40 nm is increased. It is considered that the decrease inthe number of pores with a diameter of approximately 100 nm or more isbecause the pores of the particles of the active material are coveredwith the paste-like conductive carbon. Moreover, the pores with adiameter within the range of 5 to 40 nm, which are presumably pores inthe dense, paste-like conductive carbon that is derived from theoxidized carbon, are of a sufficiently large size to allow for theelectrolytic solution in an electric storage device to go through thepaste-like conductive carbon to contact the active material particles.Therefore, it is concluded that the paste-like conductive carbon in theelectrode does not inhibit the impregnation of the electrolytic solutionin the electric storage device.

FIG. 5 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 4, Example 5 andComparative Example 5. The lithium ion secondary battery of Example 5shows a higher capacity than the lithium ion secondary battery ofComparative Example 5, and the lithium ion secondary battery of Example4 shows a higher capacity than the lithium ion secondary battery ofExample 5. That is, as the electrode density of the positive electrodeincreases, the discharge capacity per volume also increases. Also, thesesecondary batteries show almost the same rate characteristics. Thisreveals that the paste-like conductive carbons contained in the activematerial layers in the secondary batteries of Example 4 and Example 5,which are derived from the oxidized carbon and have become dense, havesufficient electroconductivity to serve as a conductive agent and do notinhibit the impregnation of the electrolytic solution in the secondarybattery. Also, the positive electrode of the secondary battery inExample 4 shows a higher electrode density than the positive electrodeof the secondary battery in Example 5, even though the contained amountof the particles of the active material and the contained amount ofcarbon in the active material layer are almost the same, which ispresumably because the fine particles are pushed out into and fill thegaps formed between the adjacent gross particles together with thepast-like oxidized carbon of Example 1 while pressing the oxidizedcarbon.

For the lithium secondary batteries of Example 5 and Comparative Example5, charging/discharging was repeated within the range of 4.6 to 3.0 Vunder the condition of 60° C. and the charging/discharging rate of 0.5C. FIG. 6 shows the result of the cycling characteristics obtained. Theresult shows that the secondary battery of Example 5 has better cyclecharacteristics than the secondary battery of Comparative Example 5.From a comparison of FIG. 2 and FIG. 3, it is considered that this isbecause almost all of the surface of the particles of the activematerial in the active material layer in Example 5 is covered with thedense paste-like carbon up to the point of the depth of the pores on thesurface of the particles of the active material and that this paste-likecarbon inhibits the degradation of the active material.

(ii) Active Material: LiCoO₂

Example 6

Li₂CO₃, Co(CH₃COO)₂ and C₆H₈O₇.H₂O were introduced into distilled water,the compound fluid obtained was agitated by a stirrer for 1 hour, andthen the compound fluid was evaporated, dried and solidified at 100° C.in air and was then heated at 800° C. in air for 10 minutes, and LiCoO₂fine particles with an average diameter of 0.5 μm were obtained. Thesefine particles and the oxidized carbon obtained in Example 1 were mixedat the mass ratio of 90:10, and a preliminary mixture was obtained.Then, 86% by mass of the total mass of commercially available LiCoO₂gross particles (average particle diameter: 10 μm), 9% by mass of thepreliminary mixture and 2% by mass of acetylene black (primary particlediameter: 40 nm) were mixed, and then 3% by mass of polyvinylidenefluoride and an adequate quantity of N-methyl pyrrolidone were added andkneaded sufficiently so that a slurry was formed, and this slurry wascoated on an aluminum foil, dried and then given a rolling treatment,and a positive electrode with an active material layer for a lithium ionsecondary battery was obtained. The electrode density of the positiveelectrode was calculated from the measured values of the volume andweight of the active material layer on the aluminum foil in the positiveelectrode. The value of the electrode density was 4.25 g/cc. Further,using the positive electrode obtained, a lithium ion secondary batterywas manufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

Example 7

94% by mass of commercially available LiCoO₂ particles (averagediameter: 10 μm), 2% by mass of the oxidized carbon of Example 1 and 2%by mass of acetylene black (primary particle diameter: 40 nm) weremixed, and then 2% by mass of polyvinylidene fluoride and an adequatequantity of N-methyl pyrrolidone were added and kneaded sufficiently sothat a slurry was formed, and this slurry was coated on an aluminumfoil, dried and then given a rolling treatment, and a positive electrodewith an active material layer for a lithium ion secondary battery wasobtained. The electrode density of the positive electrode was calculatedfrom the measured values of the volume and weight of the active materiallayer on the aluminum foil in the positive electrode. The value of theelectrode density was 4.05 g/cc. Further, using the positive electrodeobtained, a lithium ion secondary battery was manufactured in which 1MLiPF₆ in a 1:1 ethylene carbonate/diethyl carbonate solution was used asan electrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Comparative Example 6

94% by mass of commercially available LiCoO₂ particles (averagediameter: 10 μm) and 4% by mass of acetylene black (primary particlediameter: 40 nm) were mixed, and then 2% by mass of polyvinylidenefluoride and an adequate quantity of N-methyl pyrrolidone were added andkneaded sufficiently so that a slurry was formed, and this slurry wascoated on an aluminum foil, dried and then given a rolling treatment,and a positive electrode with an active material layer for a lithium ionsecondary battery was obtained. The electrode density of the positiveelectrode was calculated from the measured values of the volume andweight of the active material layer on the aluminum foil in the positiveelectrode. The value of the electrode density was 3.60 g/cc. Further,using the positive electrode obtained, a lithium ion secondary batterywas manufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

For the active material layer in Example 7 and the active material layerin Comparative Example 6, the material filling rates were confirmed byusing the abovementioned formulae (I) and (II). As a result, thematerial filling rate of the active material layer in Example 7 was85.6% and the material filling rate of the active material layer inComparative Example 6 was 79.1%; in the electrode containing thepaste-like conductive carbon derived from the oxidized carbon, animprovement in the filling rate of as much as 6.5% was observed.

FIG. 7 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 6, Example 7 andComparative Example 6. In line with the result shown in FIG. 5, FIG. 7shows that the discharging capacity increases as the electrode densityincreases and almost the same rate characteristics are obtained. For thelithium ion secondary batteries of Example 7 and Comparative Example 6,charging/discharging was repeated within the range of 4.3 to 3.0 V underthe condition of 60° C. and the charging/discharging rate of 0.5 C. FIG.8 shows the result of the cycling characteristics obtained. In line withthe result shown in FIG. 6, FIG. 8 shows that the secondary battery ofExample 7 has better cycle characteristics than the secondary battery ofComparative Example 6.

(iii) Active Material: Li_(1.2)Mn_(0.56)Ni_(0.17)Co_(0.07)O₂

Example 8

1.66 g Li (CH₃COO), 2.75 g Mn(CH₃COO)₂.4H₂O, 0.85 g Ni(CH₃COO)₂.4H₂O,0.35 g Co(CH₃COO)₂.4H₂O and 200 mL distilled water were mixed, solventwas removed by using the evaporator, and a mixture was collected. Then,the mixture collected was introduced into a vibratory ball mill device,pulverization at 15 hz was conducted for 10 minutes, and an even mixturewas obtained. The mixture after pulverization was heated at 900° C. inair for 1 hour and crystals of a lithium excess solid solutionLi_(1.2)Mn_(0.56)Ni_(0.17)CO_(0.07)O₂ with an average particle diameterof 1 μm or less were obtained. 91% by mass of these crystal particlesand 4% by mass of the oxidized carbon of Example 1 were mixed, and then5% by mass of polyvinylidene fluoride and an adequate quantity ofN-methyl pyrrolidone were added and kneaded sufficiently so that aslurry was formed, and this slurry was coated on an aluminum foil, driedand then given a rolling treatment, and a positive electrode with anactive material layer for a lithium ion secondary battery was obtained.The electrode density of the positive electrode was calculated from themeasured values of the volume and weight of the active material layer onthe aluminum foil in the positive electrode. The value of the electrodedensity was 3.15 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Comparative Example 7

91% by mass of Li₁₂Mn_(0.56) Ni_(0.17) CO_(0.07)O₂ particles that wereobtained in Example 8 and 4% by mass of acetylene black (primaryparticle diameter: 40 nm) were mixed, and then 5% by mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added and kneaded sufficiently so that a slurry was formed, andthis slurry was coated on an aluminum foil, dried and then given arolling treatment, and a positive electrode with an active materiallayer for a lithium ion secondary battery was obtained. The electrodedensity of the positive electrode was calculated from the measuredvalues of the volume and weight of the active material layer on thealuminum foil in the positive electrode. The value of the electrodedensity was 2.95 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

FIG. 9 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 8 andComparative Example 7. In line with the result shown in FIG. 5, FIG. 9shows that the discharging capacity increases as the electrode densityincreases and almost the same rate characteristics are obtained. For thelithium ion secondary batteries of Example 8 and Comparative Example 7,charging/discharging was repeated within the range of 4.8 to 2.5 V underthe condition of 25° C. and the charging/discharging rate of 0.5 C. FIG.10 shows the result of the cycling characteristics obtained. As with theresult shown in FIG. 6, FIG. 10 shows that the secondary battery ofExample 8 has better cycle characteristics than the secondary battery ofComparative Example 7.

(iv) Change of Oxidized Carbon

Example 9

10 g of furnace black with pores (average primary particle diameter: 20nm, the BET specific surface area: 1400 m²/g) was added to 300 mL of 60%nitric acid, an ultrasonic wave was irradiated for 10 minutes into thefluid obtained, and the fluid was filtrated and the furnace black wasretrieved. The retrieved furnace black was washed with water three timesand then dried, so that acid-treated furnace black was obtained. 0.5 gof this acid-treated furnace black, 1.98 g Fe(CH₃COO)₂, 0.77 gLi(CH₃COO), 1.10 g C₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31 g H₃PO₄, and 120 mLdistilled water were mixed. The compound fluid obtained was agitated for1 hour by a stirrer, and then the compound fluid was evaporated, driedand solidified at 100° C. in air, and a mixture was collected. Then, themixture obtained was introduced into a vibratory ball mill device andpulverization was conducted at 20 hz for 10 minutes. The powder afterpulverization was heated at 700° C. for 3 minutes in nitrogen, and acomplex in which LiFePO₄ was supported by the furnace black wasobtained.

1 g of the complex obtained was added to 100 mL of 30% hydrochloric acidsolution, and LiFePO₄ in the complex was dissolved while the fluidobtained was irradiated by an ultrasonic wave for 15 minutes, and thenthe remaining solid body was filtered, washed by water, and dried. Apart of the solid body after drying was heated to 900° C. in air andweight loss was measured by TG analysis. The process of dissolution ofLiFePO₄ by the aforementioned hydrochloric acid solution, filtration,water washing and drying was repeated until it was confirmed that weightloss was 100%, that is, no LiFePO₄ remained, so that a oxidized carbonthat did not contain any LiFePO₄ was obtained.

Then, 0.1 g of the oxidized carbon obtained was added to 20 mL ofammonia solution with pH 11, and ultrasonic irradiation was applied for1 minute. The fluid obtained was left for 5 hours and a solid phase areawas precipitated. After the precipitation of the solid phase area, thesupernatant fluid was removed, the remaining part was dried, and theweight of the solid object after drying was measured. By subtracting theweight of the solid object after drying from the weight of the initialoxidized carbon (0.1 g) and calculating the weight ratio of thesubtracted result against the initial weight of the oxidized carbon (0.1g), the contained amount of the “hydrophilic part” in the oxidizedcarbon was evaluated. This oxidized carbon contained 13% of hydrophilicpart. The contained amount of the hydrophilic part in the furnace blackwith pores, which was used as a raw material, was only 2%.

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂particles (average diameter: 5 μm), 2% by mass of the oxidized carbonobtained, and 2% by mass of acetylene black (primary particle diameter:40 nm) were mixed. FIG. 11 shows an SEM image of the mixture obtained ata magnification of 50,000. The surface of the particles is partlycovered with a paste-like material and their outline form is not clearlyidentifiable; this paste-like material is the oxidized carbon that isobtained by oxidizing the furnace black raw material, which spreadswhile covering the surface of the particles due to the pressure duringmixing. Also, it can be observed that acetylene black with an averageprimary particle diameter 40 nm is well dispersed. It is generally saidthat fine particles easily aggregate, but by virtue of the oxidizedcarbon, aggregation of the fine particles is effectively inhibited.

Then, 2% by mass of the total mass of polyvinylidene fluoride and anadequate quantity of N-methyl pyrrolidone were added to the mixtureobtained and kneaded sufficiently so that a slurry was formed, and thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 3.80 g/cc.Further, using the positive electrode obtained, a lithium ion secondarybattery was manufactured in which a solution of 1M LiPF₆ in a 1:1ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. For the battery obtained, charging/dischargingcharacteristics were evaluated for a broad range of current densities.Also, charging/discharging was repeated within the range of 4.6 to 3.0 Vunder the condition of 60° C. and the charging/discharging rate of 0.5C.

Example 9 and Comparative Example 5 are different in terms of the kindof carbon used for a positive electrode, but otherwise the same. InExample 9, the oxidized carbon obtained from a furnace black rawmaterial with pores and acetylene black were used, whereas inComparative Example 5, only acetylene black was used. The electrodedensity of the positive electrode in Comparative Example 5 was 3.40g/cc, so the electrode density was significantly improved by using theoxidized carbon. Example 9 and Example 5 are also different in terms ofthe kind of oxidized carbon used for a positive electrode, but otherwisethe same. In Example 5, the oxidized carbon that was obtained from theKetjen Black raw material was used, whereas in Example 9, the oxidizedcarbon obtained from the furnace black raw material was used. Theelectrode density of the positive electrode in Example 5 was 3.81 g/cc,so almost the same electrode density was obtained notwithstanding thedifference in the raw materials in the oxidized carbon.

FIG. 12 shows the relationship between the rate and the dischargecapacity per volume of the positive electrode active material layer ofthe lithium ion secondary batteries in Example 9 and Comparative Example5, and FIG. 13 shows the result of the cycling characteristics of thelithium ion secondary batteries in Example 9 and Comparative Example 5.FIG. 12 shows that the discharge capacity increases as the electrodedensity increases and almost the same rate characteristics are obtained.Also, comparison of the rate characteristics of the secondary battery ofExample 5 in FIG. 5 and the rate characteristics of the secondarybattery of Example 9 in FIG. 12 reveals that almost the same ratecharacteristics are obtained notwithstanding the difference in the rawmaterials in the oxidized carbon used for the positive electrodes. FIG.13 shows that the secondary battery of Example 9 has bettercharacteristics than the secondary battery of Comparative Example 5.Also, comparison of the cycling characteristics of the secondary batteryof Example 5 in FIG. 6 and the cycling characteristics of the secondarybattery of Example 9 in FIG. 13 reveals that almost the same ratecharacteristics can be obtained notwithstanding the difference in theraw materials in the oxidized carbon used for the positive electrodes.

(3) Solubility of Active Material

As mentioned above, it is considered that the excellent cyclecharacteristics of the lithium ion secondary battery with a positiveelectrode of the present invention is because almost all the surface ofthe particles of the active material is covered with the paste-likecarbon and this paste-like carbon inhibits the degradation of the activematerial. To confirm this, the solubility of the active material wasinvestigated.

Each of the oxidized carbon in Example 1 and acetylene black was mixedwith LiFePO₄ particles with an average diameter of 0.22 μm, LiCoO₂particles with an average diameter of 0.26 μm, andLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ with an average diameter of 0.32 μm at theratio by mass of 5:95, and then 5% by mass of the total mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added and kneaded sufficiently so that a slurry was formed, andthis slurry was coated on an aluminum foil, dried and then given arolling treatment, and an electrode was obtained. By using thiselectrode and an electrolyte in which 1,000 ppm water was added to asolution of 1M LiPF₆ in a 1:1 ethylene carbonate/diethyl carbonatesolution, a coin-type battery was manufactured. In this test, fineparticles with a large specific surface area were used in order toincrease the area of the active material that contacts the electrolyticsolution. Also, 1000 ppm water was added for the purpose of conductingan accelerated test because an active material dissolves more easilywhen there is more water. This battery was left for 1 week at 60° C.,then it was disintegrated, and then the electrolyte was collected andthe amount of metal dissolved in the electrolyte was analyzed by usingan ICP emission analysis device. Table 1 shows the result obtained.

TABLE 1 decrease amount of dissolution ratio (against of activematerial/% acetylene Mn Fe Co Ni black) LiFePO₄ + acetylene black 1.52LiFePO₄ + carbon in Example 1 0.93 −39% LiCoO₂ + acetylene black 13.1LiCoO₂ + carbon in Example 1 6.05 −54% LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ +4.5 1.59 1.32 acetylene black LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ + carbon 1.91.35 0.93 −44% in Example 1

As is evident from Table 1, the paste-like conductive carbon that isderived from the oxidized carbon in Example 1 remarkably inhibits thedissolution of the active material into the electrolyte, compared withacetylene black. This is conceivably because the oxidized carbon ofExample 1, even if its active material is fine particles with an averageparticle diameter of 0.22 to 0.32 μm, effectively inhibits theaggregation of these fine particles and covers almost all the surface ofthe particles of the active material.

(4) Mixing State of Oxidized Carbon and Active Material

The following experiment was performed to confirm the mixing state of anactive material and carbon.

(i) Mixture of Fine Particles and Carbon

Each of the oxidized carbon of Example 1 and acetylene black wasintroduced into a mortar with LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ fineparticles with an average particle diameter of 0.32 μm at the ratio bymass of 20:80 and dry blending was conducted. FIG. 14 shows SEM imagesat a magnification of 50,000. It was found that, when acetylene black isused as carbon, compared with the case where the oxidized carbon ofExample 1 is used, the fine particles aggregate even under the samemixing condition. Therefore, it was found that the oxidized carbon ofExample 1 effectively inhibits the aggregation of the fine particles.

(ii) Mixture of Gross Particles and Carbon

Each of the oxidized carbon of Example 1 and acetylene black wasintroduced into a mortar with LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ grossparticles with an average particle diameter of 5 μm at the ratio by massof 4:96 and dry blending was conducted. FIG. 15 shows SEM images at amagnification of 100,000. It was found that, when acetylene black isused as carbon, the gross particles and acetylene black existseparately, but when the oxidized carbon of Example 1 is used, the grossparticles are covered by a paste-like object, and the outline form ofthe gross particles is not clearly identifiable. This paste-like objectis the oxidized carbon obtained in Example 1, which spreads whilecovering the surface of the gross particles due to the pressure ofblending. It is considered that, by a rolling treatment when anelectrode is produced, the oxidized carbon of Example 1 further spreadsin a paste-like manner and becomes dense while covering the surface ofthe particles of the active material, the particles of the activematerial approach each other, and accordingly, the past-like oxidizedcarbon is pushed out into the gaps formed between the adjacent particlesof the active material and fills the gaps densely while covering thesurface of the particles of the active material, so that the amount ofthe active material per unit volume in the electrode is increased andthe electrode density is increased.

(5) Usage of Conductive Carbon Mixture

(i) Active Material: LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂

Example 10

The oxidized carbon obtained in Example 1 and acetylene black (primaryparticle diameter: 40 nm) were introduced into a ball mill at the massratio of 1:1 and dry blended, and a conductive carbon mixture wasobtained. FIG. 16 shows a SEM image of the conductive carbon mixtureobtained, and FIG. 17 shows TEM images of the conductive carbon mixtureobtained. FIG. 16 is an image at a magnification of 50,000, FIG. 17(A)is an image at a magnification of 100,000, and FIG. 17(B) is an image ata magnification of 500,000. It reveals from the SEM image of FIG. 16 theexistence of a paste-like carbon on the surface, because the outline ofthe carbon particles is not clearly shown. Also, the TEM images of FIG.17 shows that the conductive carbon mixture is composed of a granulatedsubstance and a layered substance on the surface of the granulatedsubstance. The dashed line in FIG. 17(B) shows the surface of thegranulated substance. The granulated substance is the particles ofacetylene black, and the layered substance is a layer that is formedbecause the oxidized carbon is collapsed and attached to the surface ofthe particles of acetylene black. FIG. 17(B) shows that the layeredobject is composed of a paste-like part in which a non-granulatedamorphous carbon is linked, and a fibrous or acicular part.

Then, 4% by mass of the conductive carbon mixture obtained, 2% by massof polyvinylidene fluoride, and an adequate amount ofN-methylpyrrolidone were wet blended, and then 94% by mass ofcommercially-available LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂ particles (averageparticle diameter: 5 μm) was further added and wet blended, and a slurrywas formed. This slurry was coated on an aluminum foil, dried and thengiven a rolling treatment, and a positive electrode with an activematerial layer for a lithium ion secondary battery was obtained. Theelectrode density of the positive electrode was calculated from themeasured values of the volume and weight of the active material layer onthe aluminum foil in the positive electrode. The value of the electrodedensity was 3.81 g/cc. Also, by using the positive electrode obtained, alithium ion secondary battery was produced, in which 1M LiPF₆ in a 1:1ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. The discharging curve of the battery obtained was measuredwithin the range of 4.5 to 3.0 V under the condition of 25° C. and thedischarging rate of 0.5 C, and the direct current internal resistance(DCIR) was calculated from the voltage drop.

Example 11

4% by mass of the conductive carbon material obtained in Example 10 and94% by mass of commercially available LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂particles (average particle diameter: 5 μm) were dry blended, and then,2% by mass of polyvinylidene fluoride and an adequate amount ofN-methylpyrrolidone were wet blended and a slurry was formed. Thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 3.80 g/cc.Further, by using the positive electrode obtained, a lithium ionsecondary battery was produced and the DCIR of the battery obtained wascalculated in the same procedure as Example 10.

The active material layers of the positive electrodes in Example 10 and11 have the same composition as the active material layer of thepositive electrode in Example 5, but the order of blending eachcomponent element in the preparation process of the electrode materialis different. FIG. 18 shows a SEM image of the cross-section of thepositive electrode in Example 10 at a magnification of 25,000. The SEMimage in FIG. 18 is similar to the SEM image of the cross-section of thepositive electrode in Example 5, which is shown in FIG. 2(B). That is,the grain boundary of the carbon primary particles does not appear, thecarbon is paste-like, this paste-like carbon intrudes into the deepparts of the pores having a width of 50 nm or less of the particles ofthe active material (ie. gaps between the primary particles), and gapsare virtually absent. Also, 90% or more of the surface of the particlesof the active material contacts the paste-like carbon. Since fine carbonparticles are poorly compatible with a binder and a solvent, when anelectrode material in the form of slurry including a binder and asolvent is prepared, it is typical that electrode active materialparticles and carbon are dry blended, and then a binder and a solventare added and wet blended. However, the electrode densities of thepositive electrodes in Example 10, Example 11 and Example 5 are almostthe same and the observation results of the SEM images are similar,therefore it has been found that the conductive carbon mixture iscompatible with a binder and a solvent, and leads an electrode thatshows a similarly high electrode density even if the conductive carbonmixture is wet blended with active material particles in the presence ofa binder and a solvent.

The DCIRs of the lithium ion secondary battery in Example 5 and thelithium ion secondary battery in Comparative Example 5, which wasproduced by only using acetylene black as a carbon, were measured in thesame procedure as the procedure in Example 10, and were compared withthe DCIRs of the lithium ion secondary batteries in Example 10 andExample 11. FIG. 19 shows the result. The DCIR of the lithium ionsecondary battery in Example 5 was remarkably lower than the DCR of thelithium ion secondary battery in Comparative Example 5, which fact showsthat remarkable decrease in DCIR is achieved by using the electrode ofthe present invention. Also the DCIRs in the secondary batteries ofExample 10 and Example 11 are still lower than the DCIR in the secondarybattery of Example 5, which fact shows that the conductive carbonmixture leads to a positive electrode with an excellent conductiveproperty regardless of the blending method of this mixture and electrodeactive material particles.

(ii) Active Material: LiCoO₂

Example 12

4% by mass of the conductive carbon mixture obtained in Example 10, 2%by mass of polyvinylidene fluoride and an adequate amount ofN-methylpyrrolidone were wet blended, and then 94% by mass ofcommercially available LiCoO₂ particles (average particle diameter: 10μm) were further added and wet blended, and a slurry was formed. Thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 4.05 g/cc.

Example 13

4% by mass of the conductive carbon mixture obtained in Example 10 and94% by mass of commercially available LiCoO₂ particles (average particlediameter: 10 μm) were dry blended, and then, 2% by mass ofpolyvinylidene fluoride and an adequate amount of N-methylpyrrolidonewere wet blended and a slurry was formed. This slurry was coated on analuminum foil, dried and then given a rolling treatment, and a positiveelectrode with an active material layer for a lithium ion secondarybattery was obtained. The electrode density of the positive electrodewas calculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the positive electrode.The value of the electrode density was 4.05 g/cc.

The active material layers of the positive electrodes in Example 12 and13 have the same composition as the active material layer of thepositive electrode in Example 7, but the order of blending eachcomponent element in the preparation process of the electrode materialis different. As the electrode densities in the positive electrodes ofExample 12, Example 13 and Example 7 are the same, it has been foundthat the conductive carbon mixture is compatible with a binder and asolvent, and leads an electrode that shows a similarly high electrodedensity even if the conductive carbon mixture is wet blended with activematerial particles in the presence of a binder and a solvent.

(iii) Change of Carbon to be Blended with Oxidized Carbon

Example 14

The oxidized carbon obtained in Example 1 and vapor grown carbon fiber(average fiber diameter: 150 nm, average fiber length: 3.9 μm) wereintroduced into a ball mill at the mass ratio of 1:1 and dry blended,and then a conductive carbon mixture was obtained. Then, 4% by mass ofthe conductive carbon mixture obtained, 2% by mass of polyvinylidenefluoride and an adequate amount of N-methylpyrrolidone were wet blended,and then 94% by mass of commercially availableLiNi_(0.5)Mn_(0.3)CO_(0.2)O₂ particles (average particle diameter: 5 μm)were further added and wet blended, and a slurry was formed. This slurrywas coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 3.66 g/cc.

Comparative Example 8

4% by mass of the vapor grown carbon fiber used in Example 14, 2% bymass of polyvinylidene fluoride and an adequate amount ofN-methylpyrrolidone were wet blended, then 94% by mass of commerciallyavailable LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂ particles (average particlediameter: 5 μm) were further added and wet blended, and a slurry wasformed. This slurry was coated on an aluminum foil, dried and then givena rolling treatment, and a positive electrode with an active materiallayer for a lithium ion secondary battery was obtained. The electrodedensity of the positive electrode was calculated from the measuredvalues of the volume and weight of the active material layer on thealuminum foil in the positive electrode. The value of the electrodedensity was 3.36 g/cc.

Comparison of Example 14 and Comparative Example 8 shows that theelectrode density is significantly improved by using the conductivecarbon mixture containing the oxidized carbon obtained in Example 1.

Example 15

The procedure of Example 14 was repeated except that graphene (planedirection length: 2 μm, cross direction length: several nm) was usedinstead of the vapor grown carbon fiber. The value of the electrodedensity was 3.69 g/cc.

Comparative Example 9

The procedure of Comparative Example 8 was repeated except that thegraphene that was used in Example 15 was used instead of the vapor growncarbon fiber. The value of the electrode density was 3.45 g/cc.

Comparison of Example 15 and Comparative Example 9 shows that theelectrode density is significantly improved by using the conductivecarbon mixture containing the oxidized carbon obtained in Example 1.

Example 16

The procedure of Example 14 was repeated except that furnace black(average particle diameter: 35 nm) was used instead of the vapor growncarbon fiber. The value of the electrode density was 3.76 g/cc.

Comparative Example 10

The procedure of Comparative Example 8 was repeated except that thefurnace black that was used in Example 16 was used instead of the vaporgrown carbon fiber. The value of the electrode density was 3.42 g/cc.

Comparison of Example 16 and Comparative Example 10 shows that theelectrode density is significantly improved by using the conductivecarbon mixture containing the oxidized carbon obtained in Example 1.

Example 17

The procedure of Example 14 was repeated except that graphite (averageparticle diameter: 6 μm) was used instead of the vapor grown carbonfiber. The value of the electrode density was 3.81 g/cc.

Comparative Example 11

The procedure of Comparative Example 8 was repeated except that thegraphite that was used in Example 17 was used instead of the vapor growncarbon fiber. The value of the electrode density was 3.48 g/cc.

Comparison of Example 17 and Comparative Example 11 shows that theelectrode density is significantly improved by using the conductivecarbon mixture containing the oxidized carbon obtained in Example 1.

INDUSTRIAL APPLICABILITY

By using the electrode of the present invention, an electrode devicewith a high energy density can be obtained.

1-12. (canceled)
 13. An electrode for an electric storage device havingan active material layer comprising: an electrode active materialparticle; and a paste-like conductive carbon that is derived from anoxidized carbon obtained by giving an oxidizing treatment to a carbonraw material with an inner vacancy and covers a surface of the electrodeactive material particle.
 14. The electrode according to claim 13,wherein the paste-like conductive carbon exists inside a gap that isformed between the electrode active material particles adjacent to eachother and/or a pore that exists on the surface of the electrode activematerial particle, the gap and the pore having a width of 50 nm or less.15. The electrode according to claim 13, wherein the active materiallayer has a pore with a diameter of 5 to 40 nm.
 16. The electrodeaccording to claim 14, wherein the active material layer has a pore witha diameter of 5 to 40 nm.
 17. The electrode according to claim 13,wherein the oxidized carbon comprises a hydrophilic part, and thecontained amount of the hydrophilic part is 10% by mass or more of theentire oxidized carbon.
 18. The electrode according to claim 14, whereinthe oxidized carbon comprises a hydrophilic part, and the containedamount of the hydrophilic part is 10% by mass or more of the entireoxidized carbon.
 19. The electrode according to claim 13, wherein 80% ormore of the surfaces of the electrode active material particles contactsthe paste-like conductive carbon.
 20. The electrode according to claim14, wherein 80% or more of the surfaces of the electrode active materialparticles contacts the paste-like conductive carbon.
 21. The electrodeaccording to claim 13, wherein the electrode active material particlesare composed of fine particles with an average diameter of 0.01 to 2 μmthat are operable as a positive electrode active material or a negativeelectrode active material and gross particles with an average diameterof more than 2 μm and not more than 25 μm that are operable as an activematerial of the same electrode as the fine particles.
 22. The electrodeaccording to claim 14, wherein the electrode active material particlesare composed of fine particles with an average diameter of 0.01 to 2 μmthat are operable as a positive electrode active material or a negativeelectrode active material and gross particles with an average diameterof more than 2 μm and not more than 25 μm that are operable as an activematerial of the same electrode as the fine particles.
 23. The electrodeaccording to claim 13, wherein the active material layer furthercomprises a different conductive carbon, and a surface of the differentconductive carbon is covered by the paste-like conductive carbon. 24.The electrode according to claim 14, wherein the active material layerfurther comprises a different conductive carbon, and a surface of thedifferent conductive carbon is covered by the paste-like conductivecarbon.
 25. The electrode according to claim 13, wherein the ratio bymass of the electrode active material particle and the conductive carbonin the active material layer is within the range of 95:5 to 99:1. 26.The electrode according to claim 14, wherein the ratio by mass of theelectrode active material particle and the conductive carbon in theactive material layer is within the range of 95:5 to 99:1.
 27. A methodof producing the electrode according to claim 13, comprising: apreparation process of blending the electrode active material particleand the oxidized carbon obtained by giving an oxidizing treatment to acarbon raw material with an inner vacancy so that an electrode material,in which at least part of the oxidized carbon is gelatinized andattached to the surface of the electrode active material particle, isprepared; and a pressure process of forming the active material layer ona current collector with the electrode material and applying pressure tothe active material layer.
 28. The method of producing the electrodeaccording to claim 27, wherein the electrode material further comprisesa different conductive carbon, and the preparation process comprises: astep of dry blending the oxidized carbon and the different conductivecarbon so that a conductive carbon mixture, in which at least part ofthe oxidized carbon is gelatinized and is attached to the surface of thedifferent conductive carbon, is obtained, and a step of dry blending orwet blending the conductive carbon mixture and the electrode activematerial particle so that the oxidized carbon, at least part of which isgelatinized, is also attached to the surface of the electrode activematerial particle.
 29. A conductive carbon mixture for producing anelectrode for an electric storage device, wherein the conductive carbonmixture comprises: a conductive carbon, at least part of which isgelatinized, which is derived from an oxidized carbon obtained by givingan oxidizing treatment to a carbon raw material with an inner vacancy;and a conductive carbon different from the oxidized carbon, and theconductive carbon, at least part of which is gelatinized, is attached toa surface of the different conductive carbon.
 30. An electric storagedevice with the electrode according to claim
 13. 31. An electric storagedevice with the electrode according to claim
 14. 32. An electric storagedevice with the electrode according to claim 19.